Antenna coupling circuit



'April 26, 1938. M. THoMPsoN ANTENNA COUPLING CIRCUIT Filed Feb. 27.1955 To FEITAHF'LIFIER u qkm BIAS E m n M A F R J Patented Apr. 26, 1938UNITED STATES PATENT OFFICE ANTENNA COUPLING CIRCUIT ApplicationFebruary 27, 1935, Serial No. 8,537

6 Claims.

rFnis invention relates to antenna coupling devices for radio receivers,particularly those of the superheterodyne type. One of the principalfeatures of this invention is an antenna coupling y circuit which may beused for the reception of wave signals in two or more frequency bandsand which eliminates the necessity of providing wave band switchingmeans in the antenna circuit.

A further feature of the invention is the provision of a circuit havingsubstantially uniform gain over two or more frequency bands.

A still further feature of the invention is the provision of meanswhereby wave signals having a frequency the same as` or close to theintermediate frequency may be prevented from being transferred from theantenna to the first radio frequency amplifier tube. i

Other features` and objects will appear hereinafter. In the drawing,Fig. 1 is a schematic diagram of a portion of a superheterodynemultiband radio receiving system embodying the invention; and

Fig. 2 is a similar diagram of a modification.

The device of Fig. 1 comprises a series resonant circuit having aninductance L1 and a capacitance C1 connected between the antenna andground.

This series circuit is shunted by a second inductance L2, said secondinductance being loosely coupled inductively to a plurality of tunedcircuits 30 A and B, although, of course, there may be more than twosuch circuits. Each circuit A or B comprises an inductance and parallelvariable capacitance and may be tuned by means of the Variable condenserto the frequency of a wave signal which 35 it is desired to receive.Circuit A may be tuned to any frequency in one wave band and circuit Bto any frequency within a second different wave band.

One side of circuit A and one side of circuit B 40 are connected to aconventional waveband switch S so that the control grid of the first R.F. amplifier tube may be connected to circuit A or circuit B in themanner well known in the art. Proper grid bias, which may be a fixedbias or the con- 45 ventional automatic volume control bias, may beapplied to the grid of this tube by connecting .'the second sides ofcircuits A and B to the supply means of the grid bias. A circuit of thetype A or B is supplied for each wave band that it is 5() desired toreceive. It is, of course, understood that if desired, only one variablecondenser may be used in which case the change in wave band isaccomplished by switching over the inductance, as shown in Fig. 2. Inthe latter case, circuit A 55 includes the variable condenser and theentire (Cl. Z50-20) inductance, Whereas cir-cuit B includes the samevariable condenser and only part of the inductance.

Thus when it is desired to receive signals in one wave band only onetuned circuit, for eX- 5 ample, circuit A, is connected to theamplifier. And when it is desired to receive a signal in another waveband, another tuned circuit, for example, B, is used instead of A.

It has been found desirable to select a value of 10 L2 such that itsresonant frequency with the minimum expected capacitance of the antennais slightly below the lowest frequency in the particular frequency bandthat it is desired to receive. By doing so, the gain throughout the 15band range may be made more uniform. Ordinarily, the gain tends toincrease with frequency. However, by selecting a parallel resonantfrequency for the antenna circuit just below the lowest frequency of theband, in the lower part of the range the coupling circuit will beoperating near resonance with a subsequent increase in gain in thatregion which will make the gain more uniform throughout the entireregion. Consequently, in a multi-band receiver, it is necessary to varythe value of L2 for each wave band in order that its resonant frequencywith the antenna capacitance may be properly adjusted for each set ofconditions. By this invention, means are provided whereby it isunnecessary to change the value of L2.

The series circuit L1, C1 may conveniently be tuned to the intermediatefrequency and, as the impedance of this unit to wave signals of or nearthat frequency will approach Zero, no signal energy of that frequencywill be transferred to the following stage.

Above the intermediate frequency,l the series unit will act as aninductive reactance in parallel with the inductance L2. The unit,therefore, 40 may be designed so that the resonant frequency of thecombined inductance and distributed capacitance of the antenna will beslightly below the lowest frequency of the range B which it is desiredto receive. Below the intermediate frequency, the series unit will actas a capacitance and thus provide a second resonant frequency for theunit comprising the distributed capacitance of the antenna, the parallelcapacitance of the series circuit, and the inductance L2 and, by 50proper design, this resonant frequency may be located slightly below thelowest frequency which it is desired to receive in range A. Thus, itwill be seen that uniform gain may be obtained in both ranges A and B byhaving the resonant frequencies of the antenna unit slightly below thelowest frequency of each range. It is, of course, necessary thattheseries resonant frequency of the series or trap circuit be located inthe region between range B and range A.

Considering the impedance function of the shunt related elements LiC1L2and the antenna, it will be seen that the impedance will be the maximumat two different frequencies, the lower frequency being that frequencyat which the capacitance of the series unit LiCi in shunt with theantenna capacity resonates with the inductance L2 and the higherfrequency being that frequency at which the series unit acts as aninductance in shunt with L2 and resonates it with the distributedcapacitance of the antenna. At the frequency at which the series circuitLiCi is resonant, the impedance of the entire unit will be a minimum.

A further advantage of this circuit is that it will minimize the effectof variations in the antenna capacity which may be caused, for example,by the swaying of the antenna with the wind. The antenna capacitance isimportant principally in determining the resonant frequency of theantenna circuit as a unit. Above the series resonant frequency of thecircuit L1C1, said circuit will act as an inductive reactance which willincrease with frequency. Thus the combined inductive reactance of thiscircuit and L2 will increase with frequency until the effect of C1becomes negligible. Consequently for one value of antenna capacitance,the unit will have one resonant frequency, but if the antennacapacitance is halved, the new resonant frequency will not be muchhigher for with a slight increase in frequency, the combined inductanceof LiCi and L2 will increase enough to make up for the change in antennacapacitance. Thus by means of this circuit the effect of variations inantenna capacitance may be minimized.

Thus, it will be seen that by this invention, means have been providedwhereby the transfer of wave signals of the intermediate frequency orthereabouts may be prevented, high and uniform gain may be provided intwo or more bands, and the necessity for providing switching means tochange the value of the conventional antenna inductance coil has beeneliminated. It will be understood that the invention is susceptible tomodifications such as will occur to persons skilled in the art.

I claim:

1. In a multi-band radio receiving system, the combination of a singleprimary winding, tunable means including at least one secondary windingassociated with said primary winding, means for adapting said tunablemeans for reception of signals in a plurality of Wave bands, an elementhaving capacitance connected to said primary winding, and a seriescircuit comprising an inductance and a capacitance in shunt relationwith said primary winding, the impedance function of said shunt relatedelements being characterized by having a plurality of points of maximumimpedance, said points occurring at frequencies in the lower portions ofthe respective wave bands, and a point of minimum impedance occurring ata frequency between said bands, said inductance and said capacitancebeing serially resonant at said last named frequency.

2. In a multi-band superheterodyne radio receiving system, thecombination of a single primary winding, tunable means including atleast one secondary winding associated with said primary winding, meansfor adapting said tunable means for reception of signals in a pluralityof wave bands, an element having capacitance connected to said primarywinding, and a series circuit comprising an inductance and a capacitancein shunt relation with said primary winding, the impedance function ofsaid shunt related elements being characterized by having a plurality ofpoints of maximum impedance, said points occurring at frequencies in thelower portions of the respective wave bands, and a point of minimumimpedance occurring at the superheterodyne intermediate frequency, saidinductance and said capacitance being serially resonant at thesuperheterodyne intermediate frequency, said frequency being locatedbetween said wave bands.

3. In a multi-band radio receiving system, the combination of a singleprimary winding, a secondary winding associated with said primaryWinding, a tuning condenser associated with said secondary winding,means for selectively includ- -ing different portions of said secondarywinding in circuit with said condenser, whereby signals in a pluralityof Wave bands may be received, an element having capacitance connectedto said primary winding, and a series circuit comprising an inductanceand a capacitance in shunt relation with said primary winding, theimpedance function of said shunt related elements being characterized byhaving a plurality of points of maximum impedance, said points occurringat frequencies in the lower portions of the respective wave bands, and apoint of minimum impedance occurring at a frequency between said bands,said inductance and said capacitance being serially resonant at saidlast named frequency.

4. In a multi-band superheterodyne radio receiving system, thecombination of a single primary winding, a secondary winding associatedwith said primary Winding, a tuning condenser associated with saidsecondary winding, means for selectively including different portions ofsaid secondary winding in circuit with said condenser, whereby signalsin a plurality of wave bands may be received, an element havingcapacitance connected to said primary winding, and a series circuitcomprising an inductance and a capacitance in shunt relation with saidprimary winding, the impedance function of said shunt related elementsbeing characterized by having a plurality of points of maximumimpedance, said points occurring at frequencies in the lower portions ofthe respective wave bands, and a point of minimum impedance occurring atthe superheterodyne intermediate frequency, said inductance and saidcapacitance being serially resonant at the superheterodyne intermediatefrequency, said frequency being located between said wave bands.

5. In a multi-band radio receiving system, the combination of anantenna, a single primary winding in series with said antenna, tunablemeans including at least one secondary winding associated with saidprimary winding, means for adapting said tunable means for reception ofsignals in a plurality of wave bands, an element having capacitanceconnected to said primary winding, and a series circuit comprising aninductance and a capacitance in shunt relation with said primarywinding, the impedance function of said antenna and said shunt relatedelements being characterized by having a plurality of points of maximumimpedance, said points occurring at frequencies in the lower portions ofthe respective Wave bands, and a point of minimum impedance occurring ata frequency between said bands, said antenna, said inductance, and saidcapacitance being serially resonant at said last named frequency.

6. In a multi-band superheterodyne radio receiving system, thecombination of an antenna, a single primary Winding in series with saidanten na, tunable means including at least one secondary windingassociated with said primary Wind* ing, means for adapting said tunablemeans for reception of signals in a plurality of wave bands, an elementhaving capacitance connected to said primary Winding, and a seriescircuit comprising an inductance and a capacitance in shunt relationwith said primary winding, the impedance function of said antenna andsaid shunt related elements being characterized by having a plurality ofpoints of maximum impedance, said points occurring at frequencies in thelower portions of the respective Wave bands, and a point of minimumimpedance occurring at the superheterodyne intermediate frequency, saidantenna, said inductance, and said capacitance being serially resonantat the superheterodyne intermediate frequency, said frequency beinglocated between said Wave bands.

MILTON L. THOMPSON.

